1. FIELD OF THE INVENTION
This invention relates to a method and an apparatus for heat-bonding optical fibers, and more specifically to a method and an apparatus for heat-bonding optical fibers in order to obtain light-conducting plates of reduced image-distortion and good air-tightness.
2. DESCRIPTION OF THE PRIOR ART
Light-conducting plates produced by heat-bonding a number of optical fibers are used, for example, as a faceplate of a cathode ray tube in the production of images, and the light-conducting plates for use as faceplates must have high airtightness, and reduced image distortion, and must be free from points of contamination impervious to light.
The conventional method for producing such a light-conducting plate comprises aligning optical fibers in good order in a mould with open ends, and heating the fibers under pressure thereby to bond the fibers to one another while permitting the air present in the interspaces between the fibers to escape from the open ends of the mould. According to such a method, however, the optical fibers are bent because the ends of the moulds are open. This causes a distortion of the image in the resulting light-conducting plate. Furthermore, if a pressure sufficient for producing an air-tight light-conducting plate having a thickness of, for example, 5 mm is applied, 40 to 50% of the optical fibers used are lost as a result of flowing out from the open ends of the mould. With a view to remedying this defect, various previous attempts have been made.
For example, U.S. Pat. No. 3,485,609 discloses a method which comprises closely packing optical fibers into a mould made of a material having a lower coefficient of expansion than the optical fibers, sealing the mould, and then heating the mould thereby to bond the optical fibers to one another by the difference in the coefficient of thermal expansion between the mould and the optical fibers. However, this method has defects. Firstly, since air present in the interspaces between the aligned optical fibers has difficulty dissolving in the glass constituting the optical fibers even upon application of pressure to the optical fibers, bubbles remain causing a deformation of the optical fibers. Furthermore, contaminated points impervious to light tend to be formed in the light-conducting plates produced by this method. Secondly, the mould must be made of a material having a low coefficient of thermal expansion and high strength at high temperatures. Moreover, in order for the mould to withstand high pressures, the mould should be made thick, and this involves a high cost of production.
Another prior art technique disclosed in U.S. Pat. No. 3,224,851 comprises packing optical fibers in a glass tube, heating the glass tube and evacuating air from it, and thereafter causing a high hydrostatic pressure to act on the peripheral surface of the glass tube thereby to bond the optical fibers. This method is also not free from disadvantages. A first drawback is that because of the need for applying a high fluid pressure at high temperatures to the glass tube packed with optical fibers, the pressure vessel should be of a material that is resistant to high temperatures and high pressures, and the equipment becomes very expensive. Another defect is that since the optical fibers flow toward the opening of the tube at which evacuation is being accomplished, distortion occurs at the end portion of the bonded optical fibers.
Accordingly, it is an object of this invention to provide a method and an apparatus for heat-bonding optical fibers with high efficiency to form an air-tight light-conducting plate having reduced distortion and contamination, without the need to use such a costly apparatus as described above.